Brake systems for vehicles (e.g., aircraft, automobiles, etc.) are well known in the art. Most brake systems include a brake actuator for exerting pressure on brake material. The brake material in turn exerts a braking torque on the element to be braked (e.g., the wheel of the vehicle). The brake actuator may be hydraulic or electromechanical, for example. By selectively activating the brake actuator, a desired amount of braking torque, or force, may be applied to the element to be braked.
In the past, torque feedback has been considered desirable in braking applications to compensate for various effects. For example, brake systems for vehicles have included a controller which utilizes the measured braking torque applied to the wheel to compensate for brake fade (due to thermal effects) and grabby brakes (common with carbon brakes). A torque sensor would measure the torque applied to the wheel and the output of the torque sensor was fed back to the controller.
Various problems arose, however, as a result of the use of torque feedback. For example, due to sensor noise and physical properties of torque, the output of the torque sensor was not valid at or near zero wheel speed. To account for this, the torque feedback was disabled below a predefined wheel speed and the brake system would revert to open loop control. This "low speed cutout" of the torque feedback control would naturally have to occur at a speed at which the output of the wheel speed sensor was still valid. Since wheel speed sensors typically are valid only to a predefined lower speed limit, the low speed cutout was required to occur at a speed greater than the lower speed limit. Thus, the limitations of the torque sensor and the wheel speed sensor precluded torque compensation at low wheel speeds.
Another problem with torque feedback using low speed cutout control is that the transition from closed loop control to open loop control has to occur over a period of time. If the pilot of an aircraft or driver of an automobile happens to be activating the brake during this time, a gradual change may be felt. If the pilot or driver does not happen to be operating the brake during the transition period, however, a sudden change may be perceived in the next brake application. This sudden change would present a discontinuity in the braking felt by the pilot/driver and even passengers, creating feelings of discomfort and/or alarm.
Furthermore, problems occur when proportional-integral (P-I) controllers are used in combination with the low speed cutout. If braking is applied at high speed, the integral portion of the P-I controller will tend to overshoot and cause the brake to grab initially. This can cause a short wheel skid if the surface on which the wheel is running is not dry.
In view of the aforementioned shortcomings associated with brake systems employing torque feedback control, there is a strong need in the art for a brake system which provides more suitable torque compensation. There is a strong need in the art for a system in which allows torque compensation operation to substantially zero wheel speed. Moreover, there is a strong need for a system in which torque compensation is provided with substantially no discontinuities, regardless of time or torque level. In addition, there is a strong need for a system in which the brakes do not tend to grab as a result of brake material or the integral portion of a P-I controller.